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Investments Subject to Significant Influence and Equity Income (Equity Method) (Details) - CAD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2018 | Dec. 31, 2017
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | $ 1,316 | $ 1,215
Income (Loss) from Equity Method Investments |  | 154 | 124
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | 1,316 | 1,215
Equity Method Investment, Difference Between Carrying Amount and Underlying Equity |  |  | 13
Other Liabilities, Noncurrent |  | 686 | 609
Other Investments [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 40 | 18
Income (Loss) from Equity Method Investments |  | 4 | 2
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | 40 | 18
Labrador-Island Link Limited Partnership [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 534 | 492
Income (Loss) from Equity Method Investments |  | $ 42 | 37
Equity Method Investment, Ownership Percentage |  | 49.50% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Indirect Ownership Percentage By Parent |  | 24.90% |
Equity Method Investments |  | $ 534 | 492
NSP Maritime Link Inc. [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 545 | 510
Income (Loss) from Equity Method Investments |  | $ 45 | 36
Equity Method Investment, Ownership Percentage |  | 100.00% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | $ 545 | 510
Maritimes and Northeast Pipline [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 155 | 156
Income (Loss) from Equity Method Investments |  | $ 22 | 23
Equity Method Investment, Ownership Percentage |  | 12.90% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | $ 155 | 156
Saint Lucia Electricity Services Limited [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 42 | 39
Income (Loss) from Equity Method Investments |  | $ 3 | 3
Equity Method Investment, Ownership Percentage |  | 19.10% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Equity Method Investments |  | $ 42 | 39
Bear Swamp Power Company Limited Liability Company [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method Investments |  | 0 | 0
Income (Loss) from Equity Method Investments |  | $ 38 | 23
Equity Method Investment, Ownership Percentage |  | 50.00% |
Equity Method Investment, Summarized Financial Information [Abstract] |  |  |
Proceeds from Equity Method Investment, Dividends or Distributions, Return of Capital | $ 179 |  |
Equity Method Investments |  | $ 0 | 0
Other Liabilities, Noncurrent |  | $ 172 | $ 188

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